This invention relates generally to pressure modulation in hydrostatic drive systems alone in split torque transmissions and more particularly to methods for the rapid adjustment of hydraulic pressure in a loop including a variable displacement hydraulic pump and a fixed or variable displacement hydraulic pump motor. The invention is useful to supplement the ranges of hydrostatic system response and particularly to respond to conditions outside the intended performance envelope which may develop more rapidly than the operator or equipment are expected to be responsive to.
Hydrostatic drive systems in split torque transmission employ a pressurized hydraulic loop to connect an engine to a driven element. The engine is not specifically limited, but the preponderance of vehicles and stationary devices using hydrostatic split torque transmissions also employ internal combustion engines and, typically, diesel engines. The diesel engine through drives an hydraulic pump, preferably a variable displacement pump which provides pressurized hydraulic fluids used to operate various components of the driven system. The hydraulic pressure may be used to drive a fixed or a variable displacement slaved pump or motor. The pump or motor may be connected to a gear box drive system of a split torque transmission and provides torque to move the vehicle. Such pump motors may also be used to drive winches or other rotating equipment. Under certain circumstances, load transferred from the driven elements may be backloaded to the slaved motor and result in unplanned or undesirable pressure changes within the system. The excursions may cause overspeed of the engine, excessive torque demand or lugging of the engine depending upon the direction and degree of the backloading.
U.S. Pat. No. 4,024,775 to Anderson et al., granted May 24, 1977, is directed to an hydrostatic mechanical transmission having an infinitely variable split torque. The transmission includes two hydrostatic units which may act, interchangeably, as motor and pump. The flow paths between the two units are fixed in length and diameter.
U.S. Pat. No. 5,842,144 to Coutant et al, granted Nov. 24, 1998, discloses an electronic control system for a split torque transmission which monitors and controls, inter alia, hydrostatic pump displacement controllers. The reference teaches the monitoring and control of variable displacement hydraulic pumps in the transmission, but does not disclose methods for controlling flow rates between the pumps.
The present invention is directed to overcoming one or more of the problems as set forth above.
The invention is directed to the inclusion of a shunt or loop in parallel with the primary hydraulic loop between a variable output hydraulic pump and a fixed or a variable displacement hydraulic pump motor in the hydrostatic system of a split torque transmission. One or more xe2x80x9cvariable areasxe2x80x9d are used to control flow, one variable area being in the primary loop and a second variable area being in the shunt or parallel loop.
In a first aspect of the invention, the variable area of the shunt loop may be used as an hydraulic pressure control system to limit torque transfer by draining pressure from the primary loop.
In a second aspect of the invention, the shunt loop may be used to avoid lugging of the engine when increasing loads are placed on the engine or when very rapid load demands are made as nay occur when encountering an immovable object. The shunt loop variable area is opened reducing the load on the engine by directing hydraulic fluid through a lower resistance pathway.
In a third aspect of the invention, the variable area in the shunt loop may be used, in combination with a variable area in the primary loop, to avoid backloading to the engine by limiting the flow rate from a slaved motor on the load side of the loop to the variable displacement pump at the engine side of the loop. Reduction in the backloading to the engine prevents overspeed of the engine and of the variable output pump.
The xe2x80x9cvariable areasxe2x80x9d according to this invention may be any flow restriction device or throttle which can be mechanically, hydraulically, pneumatically or electrically controlled. Activation is under the control of hydraulic line pressure and electronic signals obtained from the engine and other locations including those identified in U.S. Pat. No. 5,842,144.